Concentration of aromatic hydrocarbons with diluted sulfur dioxide



Oct. 3, 1961 Filed Sept. 14, 1959 A. w. FRANCIS 3,003,006 CONCENTRATION OF AROMATIC HYDROCARBONS WITH DILUTED SULFUR DIOXIDE 2 Sheets-Sheet 1 Nonaromatic Hydrocarbons Ethylene Glycol F l or Formamide v Aromatic Hydrocarbons (D S0 0 COMPRESSOR x (LIQUID) g q, so (GAS) 2 S 2 3 N '8 w ..2 2 3 2 A g 23? t c O 6 m 3 w PUMP 7 7 q 32 B 5 g 5 E 0 (II o O O m .r: E E 2 o f a O 3. n. 8 m u. z I "u E 2 c I O o a: o O O 5 E '3 0 E w 2 F, I o W E 24 g 2 9 g z 6 O O E 3 Z 25 9 a 2 a *5 5 Reflux E Aromatic g 9) Hydrocarbons 8 l7 g 5 o g PUMP J \x E 6 Ethylene Glycol f 5 I Aromatic Hydrocarbons 3 8t Sulfur DlOXlde }g j,

Y Ethylene Glycol or Formamide satd. with Aromatic Hydrocarbons 5 Feed INVENTOR. Lighl' Aromatic 8 Alfred W. Francis PUMP Nonaromatic Hydrocarbons Ethylene Glycol or Formamide ATTORNEY By/gzmdtaw.

Oct. 3, 1961 A. w. FRANCIS 3,003,005

CONCENTRATION OF AROMATIC HYDROCARBONS WITH DILUTED SULFUR DIOXIDE Filed Sept. 14. 1959 2 Sheets-Sheet 2 Benzene 75%80 n-Heptone Ethylene Glycol Benzene 25% Ethylene Glycol or Formomide Formomide Benzene Ethylene Glycol S0 Ethylene Glycol FlG.6

Diethylene Glycol Benzene Woter I $0 INVENTOR.

Alfred W. Francis-- v WM 0 ATTORN EY United States This invention relates to the concentration of aromatic hydrocarbons from a stream containing both aromatic and nonaromatic hydrocarbons with diluted sulfur dioxide.

Sulfur dioxide has long been recognized as an excellent solvent for aromatic hydrocarbons in solvent refining of petroleum products. This material is low in cost and requires only moderate pressure at operating temperatures to maintain it in liquid form, thereby simplifying the pumping operations. However, sulfur dioxide has the disadvantage that its solvent power varies steeply with the molecular Weight of the hydrocarbons being separated and hence this fact has made sulfur dioxide of limited utility for the refining of lubricating oil. While sulfur dioxide dissolves an excessively small fraction of heavy oil, it mixes completely with many nonaromatic hydrocarbons boiling in the gasoline boiling range, such as hexane, heptane, cyclohexane and methylcyclohexane.

For this reason sulfur dioxide alone cannot be used at ordinary temperatures to concentrate light aromatic hydrocarbons, such as benzene, toluene, etc. from hydro-' carbon mixtures. The prior art teaches the use of sulfur dioxide as an extraction solvent for aromatic hydrocarbons by operation at low temperatures down to 60 F. This process is disclosed in Chemical Refining of Petroleum, by Kalichevsky and Stagner, second edition, pp. 320--l (1942). This process is also disclosed in U.S. Patent No. 2,758,141. Unfortunately, the process is not satisfactory because solid phases at the low temperatures interfere with the operation of the process, and the refrigeration costs necessitated for operation at such low temperatures are prohibitive and make the process uneconomical for commercial use.

Another method proposed for utilizing sulfur dioxide as a solvent extraction agent is found in the US. Patents 2,410,166 and 2,724,682. These processes involve mixing a wash oil or high molecular weight paraflinic oil with the hydrocarbon oil to be separated, the parafiinic oil being practically immiscible with the sulfur dioxide. While this type of operation can be used to concentrate aromatic hydrocarbons, it is exceedingly expensive and is hence commercially impractical for that reason.

The prior art has recognized that no satisfactory diluting agent for sulfur dioxide has so far been found, as evidenced by the U8. Patent No. 2,396,299, which states that unsuccessful experiments have been made to modify sulfur dioxide and states further that even with the use of modifying agents, it is subject to other limitations in the purity of extract obtainable, even at very low temperatures. It is further stated in the above patent: No wholly satisfactory solvent has yet been found for changing the dissolving power of liquid sulfur dioxide without impairment of its selectivity due to the properties of sulfur dioxide.

I have found that, contrary to prior belief, the solvent power of sulfur dioxide can be controlled successfully to provide a mixed solvent capable of efficiently concentrating light aromatic hydrocarbons from a mixture of light aromatic hydrocarbons and nonaromatic hydroatnt 2 carbons boiling at substantially the same boiling point by mixing with the sulfur dioxide an effective amount of a material selected from the group consisting of ethylene glycol and formamide. I have provided a decidedly improved process for concentrating light aromatic hydrocarbons in a mixture of aromatic and nonaromatic hydrocarbons using a mixed solvent of sulfur dioxide and a material selected from the group consisting of ethylene glycol and formamide.

Water cannot be used for dilution of sulfur dioxide because Water has a very low solubility in sulfur dioxide. The solubility of water in sulfur dioxide is less than 0.5% up to 30 C., 1% at 60 (3., and only 2% at C. (Torres and Riihl, Z. angew. Chem. 47, 331 (1934); Landolt-Bornstein-Roth-Scheel, Tabellen, 3rd Erg. (a) 672). These small amounts of water in the sulfur dioxide phase have little effect on the miscibility of sulfur dioxide with hydrocarbons. Thus, sulfur dioxide saturated With water mixes with n-heptane above 28 C. as compared with 19 C. for anhydrous sulfur diom'de, a trifling increase as concerns solvent extraction techniques. Glycerol, like water, is almost insoluble in sulfur dioxide, raising the critical solution temperature with n-heptane only to 23 C.

On the other hand, exhaustive study has demonstrated that most liquids miscible with sulfur dioxide have also a considerable solvent power for light aromatic hydrocarbons. Thus, a saturated solution of benzene in diethylene glycol (the Udex solvent) contains 31% benzene by weight (see Johnson and Francis, Ind. Eng. Chem. 46, 1662 [1954]). Further, a saturated solution of benzene in formic acid or in propylene glycol contains about 13% by weight benzene at standard temperature of 60 F. Such large concentrations of aromatic hydrocarbons in the solvent make such combined solvents impractical as a solvent for the solvent extraction of aromatic hydrocarbons from streams containing both aromatic and nonaromatic hydrocarbons.

The diluent for sulfur dioxide must be completely miscible with sulfur dioxide. This requirement excludes both water and glycerol. The diluent must be liquid at desired operating temperature, such as 0 to 50 C. This requirement excludes urea, acetamide, maleic anhydride and other solvents which are not liquid in the desired operating temperature range. The diluent must be unreactive with sulfur dioxide, which eliminates eth anolamines, etc. The diluent should be almost immiscible with light aromatics, such as benzene, toluene, etc., so as to minimize the amount of the aromatic hydrocarbons to be scrubbed out of the recirculated diluent. This requirement excludes diethylene glycol, propylene glycol, formic acid, etc., which dissolve between about 13-61% benzene. The only liquids found to satisfy these as well as other requirements are ethylene glycol and formamide.

The object of this invention is to provide a process for separating aromatic hydrocarbons from nonaromatic hydrocarbons boiling at substantially the same boiling point.

A further object of this invention is to provide a simplified process for using the solvent power of sulfur dioxide to separate aromatic hydrocarbons from nonaromatic hydrocarbons boiling at substantially the same boiling point.

A further object of this invention is to provide an economical process for extracting light aromatic hydrocarbons from a mixture of light aromatic hydrocarbons and nonaromatic hydrocarbons at moderate temperature levels and using diluted sulfur dioxide as the solvent medium.

A further object of this invention is to provide a highly efficient continuous process for separating light aromatic hydrocarbons from nonaromatic hydrocarbons boiling at substantially the same boiling point using diluted liquid sulfur dioxide under moderate temperature levels and mildly advanced pressure as the extracting medium.

These and other objects of the invention will be more fully disclosed in the following detailed discussion of the invention, which is to be read in conjunction with the attached figures.

FIGURE 1 shows diagrammatically a complete process for using modified sulfur dioxide as an extracting solvent.

FIGURE 2 shows a ternary diagram for benzene and heptane with 75% sulfur dioxide and 25% of either ethylene glycol or formamide comnn'ngled therewith as a solvent medium.

FIGURE 3 shows a ternary diagram for sulfur dioxide, benzene and ethylene glycol.

FIGURE 4 shows a ternary diagram for benzene, sulfur dioxide and formamide.

FIGURE 5 shows a ternary diagram for sulfur dioxide, heptane and ethylene glycol.

FIGURE 6 shows a ternary diagram for sulfur dioxide, benzene and diethylene glycol.

FIGURE 7 shows a ternary diagram for ethylene glycol, sulfur dioxide and water.

Referring now to FIGURE 1, a method of operating the process will be described in detail. An elongated extraction column 1 is operated at a convenient temperature, such as 25 C., and the vapor pressure of sulfur dioxide at that temperature, about 50 p.s.i.g. The limits of temperature and pressure are not critical, being governed largely by economy. The pressure will range from about 25-l25 p.s.i.g., being in any case that pressure required to keep the sulfur dioxide in liquid form at the temperature used. The temperature will ordinarily range from about -50 C. The feed is a mixture of aromatic and nonaromatic hydrocarbons which, for purposes of illustration, may be a mixture of benzene and n-heptane. The mixed benzene-n-heptane feed is used to scrub benzene out of the recycle ethylene glycol or forrnamide in a recycle or second stripping zone 3. The feed is supplied to the lower portion of the stripping column 3 through the conduit and is withdrawn from the top of the stripping column 3 through the conduit 6. Makeup ethylene glycol or formamide is introduced into a first stripping column 4 at the upper portion of this stripping column through the conduit 7 and travels downwardly in counterfiow with the final aromatic hydrocarbon product. The ethylene glycol or formamide is saturated with the arcmatic hydrocarbon, and the saturated ethylene glycol is withdrawn from the bottom of the first stripping column through the conduit and introduced into the upper portion of the second stripping column 3. Both the first and second stripping columns are maintained at substantially atmospheric pressure. The feed scrubs aromatic hydrocarbons from the ethylene glycol while flowing countercurrently with the ethylene glycol in the second stripping zone 3.

The feed is pressured in the pump 9 to about 50 p.s.i.g. and passed through the conduit 10 into the lower portion of the extraction column 1. Ethylene glycol (or formamide alternatively) is withdrawn from the bottom of the second stripping column through the conduit 11 and pressured in the pump 12 to about 50 p.s.i.g. The ethylene glycol under pressure is transferred through the conduit 13 into the upper portion of the extraction column 1. Using a feed of benzene-n-heptane, the n-heptane rises upwardly through the extraction column 1 and is withdrawn through the conduit 14 at the top of the column. The ethylene glycol moves downwardly through the column and first scrubs out sulfur dioxide from the n-heptane product. The sulfur dioxide in liquid form at the advanced pressure of 50 p.s.i.g. is introduced into the column 1 through the conduit 15 at a level a substantial distance below the level'at which the ethylene glycol is introduced into the column. This insures that the ethylene glycol is provided with adequate contact time to complete the removal of sulfur dioxide from the product nheptane before the ethylene glycol is combined with the sulfur dioxide. The liquid sulfur dioxide, injected above the middle of the column; mixes with the ethylene glycol, and extracts benzene as it descends counter-current to the n-heptane stream. The composition of the mixed solvent should be about 70-75% sulfur dioxide by weight. Broadly, the ratio of sulfur dioxide to ethylene glycol (or formamidc) by weight may range from 2 to 1 up to 9 to 1. The mixture of ethylene glycol, sulfur dioxide and benzene is withdrawn from the bottom of the extraction column 1 through the conduit 16. The pressure is released by the relief valve 17 to approximately atmospheric pressure, for convenience, although other pressures could be used as desired. It is only essential that the pressure be reduced sufficiently to convert the sulfur dioxide from a liquid to a gas. The depressured mixture of ethylene glycol, sulfur dioxide and benzene are introduced into the lower portion of a settling chamber 2 where the sulfur dioxide gas rises to the top above the liquid level. In the absence of most of the sulfur dioxide, the ethylene glycol and benzene are practically immiscible and separate into two layers. The solubility of benzene in ethylene glycol or formamide is about 5% whereas the solubility of ethylene glycol or formamide in benzene is 0.07% The lower ethylene glycol layer is withdrawn from the bottom of the chamber 2 through the conduit 26 and combined with the makeup ethylene glycol in the conduit 8. The sulfur dioxide gas is withdrawn from the top of the chamber 2 through the conduit 18 and compressed in the compressor 19 to about 50 p.s.i.g., cooled in the heat exchanger 25 located in the conduit 15 and situated in the lower portion of the settling chamber 2 wherein the gas converts to a liquid. The heat supplied to chamber 2 by heat exchanger 25 provides the latent heat of vaporization of sulfur dioxide. Since the latent heat of condensation of the sulfur dioxide passed through compressor 19 must be removed so as not to raise the temperature and pressure in the extraction column 1 and since the two heats are approximately equal, the provision of heat exchanger 25 in conduit 15 eificiently effects the desired heat transfer. The liquid level in the chamber 2 is maintained a substantial distance below the top of the chamber to allow for gas separation. The benzene in the upper liquid layer is withdrawn through the conduit 26 and introduced into the lower portion of the first stripping column 4. The benzene travels upwardly through the column 4 in counterflow with the makeup ethylene glycol, and the benzene product is withdrawn from the top of the stripping column 4- through the conduit 21.

An improved operation of the process can be obtained by withdrawing a portion of the benzene from the chamber 2 through the conduit 22 as reflux. The benzene is pressured in the pump 23 to about 50 p.s.i.g. and passed through the conduit 24 into the lower portion of the extraction column 1.

It is a particular feature of this invention that only the sulfur dioxide requires distillation and that the entire process operates atornear room temperature or standard temperature of 25 C. Since neither the highboiling tion of the process, using about 75% by weight of sulfur dioxide and 25 of the cosolvent, either ethylene glycol or formamide. It is possible todilute the ethylene glycol slightly with water to reduce the cost without detriment to the solvent selectivity. For maximum selectivity the proportion of sulfur dioxide is adjusted so that the binodal curve nearly touches the left side of the ternary triangle,- as indicated on FIGURE 2. The plait point is indicated by a small circle on the binodal curve. It is, of course, well known in the prior art that certain solvents are selective in separating hydrocarbons from a mixture of aromatic and nonaronratic hydrocarbons. These solvents are miscible with the aromatic hydrocarbons and have a moderate solvent power for nonaromatics. These solvents reduce the nonarornatic content to a fixed or limiting proportion with increasing extraction stages and thereafter the portion of nonaromatic of the same boiling point as the aromatic extracted remains constant and the mixture cannot be purified to pure aromatic hydrocarbons regardless of the number of extraction stages used. This limitation can be expressed on the ternary diagram by a line drawn from the solvent corner of the triangle touching the binodal curve and extended to the opposite side of the triangle. This represents the limiting percentage of purification. It is, therefore, desirable to raise the binodal curve to touch the left side of the triangle, as indicated in FIGURE 2, so that substantially pure aromatic can be produced. The solvent mixture of sulfur dioxide and the correct proportion of ethylene glycol or formamide permits this desired result to be achieved, as shown in FIG- URE 2. The use of tenary diagrams to express graphically the conditions occurring when three liquids are mixed is disclosed in Textbook of Physical Chemistry, by Samuel Glasstone, starting at page 782.

FIGURES 3 and 4 show ternary equilibria between benzene, sulfur dioxide and the anhydrous cosolvents. They show that up to fairly high concentrations sulfur dioxide goes by preference to the solvent layer. This is, of course, desirable since it does no good in the hydrocarbon layer. When the hydrocarbon is partly nonaromatic, the preference of sulfur dioxide for the solvent layer is much greater, as shown on FIGURE 5. A most important feature of FIGURES 3 and 4 is the low solubility of benzene in the cosolvent, as indicated by the proximity of the binodal curve to the left corner of the triangle. In contrast, FIGURE 5 shows a high solubility of benzene in diethylene glycol of about 31%, even in the absence of sulfur dioxide, and much more in its presence. It would not be economically practical to recover this large amount of benzene in an extraction process.

FIGURE 7 shows ternary equilibria for water, ethylene glycol and sulfur dioxide. This diagram shows that as much as 38% water could be added to the ethylene glycol used as a diluent without separating the solvent into two layers. However, such a large concentration of water would drive most of the sulfur dioxide into the hydrocarbon layer, and diminish the solubility of the hydrocarbon in the ethylene glycol layer almost to zero. Not over 15-20% of Water based on the ethylene glycol can be tolerated. Such a limited amount of water is contemplated as within the scope of the invention, such as less than about 15% water. Small amounts of water decrease the cost of the solvent, decrease the solvent viscosity and further decrease its solvent power for aromatics. These advantages of water dilution are oifset to some extent by the fact that water decreases slightly the selectivity of the solvent. It also increases considerably the tendency of sulfur dioxide to corrode the equipment. While water may also be added to formamide, this is less desirable because of the possibile gradual hydrolysis of formamide with Water.

Actual extractions are reported in the following Tables I to VII. The final column in each table, 12 is a measure of selectivity in extraction. The value of B is found as follows:

(aromatic in extract) (parafl'in in raffinate) TABLE I Benzene and n-hexane using ethylene glycol Parts by weight Percent Ethylene glycol Extract n-Hexans Benzone TABLE II Benzene and n-heptane using ethylene glycol Parts by Weight Percent benzene n n-Hexans Benzene Extract SO; mate TABLE III Toluene and n-heptane using ethylene glycol Percent toluene in Parts by weight Remnate Extract n-Heptans Toluone Ethylene glycol TABLE IV Benzene and n-heptane using diluted ethylene glycol Parts by weight Percent toluene in- Extract n-Heptans E thylene glycol Ben- Water zone TABLE V Benzene and n=hexane using forma-mide Percent benzene iu- Parts by weight Extract Benzene HOONH n-Hex- TABLE VI Benzene and n-hepmne using formamlde Percent benzene 111- Parts by weight Extract n-Heptans S02 HG ONH: Ben- The extractions reported in Tables I-VII to illustrate the invention were single-stage extractions. In the continuous extraction process illustrated on FIGURE 1, a plurality of extraction stages is utilized, the actual number selected being based upon the desired purity of aromatic hydrocarbons produced and the desired yield of aromatic hydrocarbons.

While an exhaustive study has been made to determine equivalent diluents for sulfur dioxide, only ethylene glycol and formamide have been found satisfactory. The various materials tested and rejected included, among others, diethylene glycol, propylene glycol, ethylene cyanohydrin, formic acid, lactic acid, maleic anhydride, urea, glycerol, diethanolamine, sulfuric acid, oleum, aluminum chloride, antimony chloride, copper chloride, iron chloride, zinc chloride, hydrogen chloride, hydrogen bromide, hydrogen cyanide, sulfur, sulfur hexafluoride, and carbon di-sulfide.

The illustrations of the invention given hereinabove were presented only to demonstrate the invention and are not intended as a limitation of the scope of the invention. The only limitations intended are those found in the attached claims.

I claim:

1. The method of separating light aromatic'hydrocarbons from nonaromatic hydrocarbons boiling at substantially the same boiling point which comprises contacting a mixture of light aromatic hydrocarbons and nonaromatic hydrocarbons boiling at substantially the same boiling point with a solvent comprising a mixture of sulfur dioxide and a material selected from thegroup consisting of ethylene glycol and formamide, separating the material into a rafl'inate phase containing substantially all of the nonaromatic hydrocarbons and an extract phase containing substantially all of the aromatic hydrocarbons in conjunction with the mixture of sulfur dioxide and the material selected from the group consisting of ethylene glycol and formamide, separately removing sulfur dioxide from the extract phase for reuse in the process, separately removing the material selected fromthe group consisting of ethylene glycol and formamide from the extract phase for reuse in the process, and separately removing from the extract phase as product substantially pure light aromatic hydrocarbons free of nonaromatic hydrocarbons and free of solvent materials.

2. The method of separating light aromatic hydrocarbons from nonaromatic hydrocarbons boiling'at substantially the same boiling point which comprises the steps of contacting a mixture of light aromatic hydrocarbons and nonaromatic hydrocarbons with a solvent comprising a mixture of sulfur dioxide and a material selected from the group consisting of ethylene glycol and formamide at a temperature not substantially below 25 C. and at a pressure at least sufficient to maintain the sulfur dioxidein liquid form, separating a raflinate phase containing substantially-all ofthe'nonaromatic hydrocarbons, separating an extract phase containing a mixture of sulfur dioxide, aromatic hydrocarbons and the material selected from the group consisting of ethylene glycol and formamide, reducing the pressure on the extract phase and adding suflicient heat to the extract phase to remove the sulfur dioxide from the mixture in the form of-a gas, separatingt-he light-aromatic hydrocarbons and-material selected from the group consisting-of ethylene glycol and formamide into two liquid layers by gravity separation, removing the aromatic hydrocarbons as substantially pure product, removing the material selected from the group consisting of ethylene glycol and formamide, repressuring said material and recycling said material for reuse in the process, rcpressuring and cooling said sulfur dioxide to convert said sulfur dioxide to a liquid and recycling said liquid sulfur dioxide for reuse in the process.

3. Claim 2 further characterized in that the ratio of sulfur dioxide by weight to material selected from the group consisting of ethylene glycol and formamide by weight is within the ratio 2 to 1 and 9 to 1.

4. Claim 2 further characterized in that the presence maintained within the contacting zone is between about 25-425 p.s.i. g.

, 5. Claim 2 further characterized in that the selected material is ethylene glycol.

6. Claim 2 further characterized in that the selected material is formamide.

7. The method of separating light aromatic hydrocarbons from nonaromatic hydrocarbons boiling at substantially the same boiling point which comprises the steps of introducing ethylene glycol at advanced pressure into the upper portion of a contacting zone, introducing sulfur dioxide at advanced pressure and in liquid form into the upper portion of said contacting zone at a level substantially below the level at which ethylene glycol is introduced, whereby nonaromatic hydrocarbons rising through the contacting zone are stripped of their content of sulfur dioxidetwith ethylene glycol before reaching the top of the zone, withdrawing nonaromatic hydrocarbons from the top of the contacting zone, introducing a mixture of light aromatic hydrocarbons and nonaromatic hydrocarbons boiling at substantially the same boiling point into the lower portion of said contacting zone at advanced :pressure, to supply the aforementioned nonaromatic hydrocarbons rising through said contacting zone, withdrawing a mixture of ethylene glycol, sulfur dioxide and aromatic hydrocarbons from the bottom of the contacting zone and discharging the mixture at substantially atmospheric pressure into a separating zone, adding suificient heat to said separating zone to form a three-phase system, with gaseous sulfur dioxide at the top, liquid aromatic hydrocarbons in the middle, and ethylene glycol at the bottom, withdrawing sulfur dioxide gas from the top of the separating zone, compressing the sulfur dioxide to a pressure equal to the pressure in the contacting zone, and extracting sufficient heat from the sulfur dioxide to convert the sulfur dioxide to a liquid, and supplying the sulfur dioxide in liquid form to the contacting zone as the aforementioned sulfur dioxide, transferring aromatic hydrocarbons from the separation zone to the lower portion of a substantially atmospheric first stripping zone, introducing ethylene glycol into the upper portion of said first substantially atmospheric stripping zone, withdrawing light aromatic hydrocarbons from the top of said first atmospheric stripping zone, withdrawing ethylene glycol from the bottom of said first substantially atmospheric stripping zone and introducing said ethylene glycol into the upper portion of a second substantially atmospheric stripping zone, introducing a mixture of light aromatic hydrocarbons and nonaromatic hydrocarbons boiling at substantially the same boiling point into the lowier portion of said second substantially atmospheric stripping zoue, withdrawing the mixture of light aromatic and ;11onaro1natic hydrocarbons from the top of said second substantially atmospheric stripping zone, compressing said mixture of hydrocarbons to a pressure equal to the pressure in said contacting zone and supplying said mixture of hydrocarbons tothe contacting zone as the aforementioned mixture of light aromatic and nonaromatic hydrocarbons, withdrawing ethylene glycol from the bottom of said-second substantially atmospheric stripping zone, compressing the ethylene glycol to the pressure in the contacting zone and supplying the ethylene glycol to the contacting zone as the aforementioned ethylene glycol.

8. Claim 7 further characterized in that a recycle stream of aromatic hydrocarbons is withdrawn from the separating zone, compressed to the pressure in said contacting zone and introduced into the lower portion of said contacting zone as a refluxing stream.

9. The method of separating light aromatic hydrocarbons from nonaromatic hydrocarbons boiling at substantially the same boiling point which comprises the steps of introducing formamide at advanced pressure into the upper portion of a contacting zone, introducing sulfur dioxide at advanced pressure and in liquid form into the upper portion of said contacting zone at a level substantially below the level at which formamide is introduced, whereby nonaromatic hydrocarbons rising through the contacting zone are stripped of their content of sulfur dioxide with formamide before reaching the top of the contacting zone, withdrawing nonaromatic hydrocarbons from the top of the contacting zone, introducing a mixture of light aromatic and nonaromatic hydrocarbons boiling at substantially the same boiling point into the lower portion of said contacting zone at advanced pressure, to supply the aforementioned nonaromatic hydrocarbons rising through said contacting zone, Withdrawing a mixture of formamide, sulfur dioxide and aromatic hydrocarbons from the bottom of the contacting zone and discharging the mixture at substantially atmospheric pressure into a separating zone, adding sufficient heat to the separating zone to form a three-phase system, with gaseous sulfur dioxide at the top, liquid aromatic hydrocarbons in the middle, and formamide at the bottom, withdrawing sulfur dioxide gas from the top of the separating zone, compressing the sulfur dioxide to a pressure equal to the pressure in the contacting zone, and extracting sufficient heat from the sulfur dioxide to convert the sulfur dioxide to a liquid, and supplying the sulfur dioxide in liquid form to the contacting zone as the aforementioned sulfur dioxide, transferring aromatic hydrocarbons from the separation zone to the lower portion of a substantially atmospheric first stripping zone, introducing formamide into the upper portion of said first substantially atmospheric stripping zone, withdrawing light aromatic hydrocarbons from the top of said first atmospheric stripping zone, withdrawing formamide from the bottom of said first substantially atmospheric stripping zone and introducing said formamide into the upper portion of a second substantially atmospheric stripping zone, introducing a mixture of light aromatic hydrocarbons and nonaromatic hydrocarbons boiling at substantially the same boiling point into the lower portion of said second substantially atmospheric stripping zone, withdrawing the mixture of light aromatic and nonaromatic hydrocarbons from the top of said second substantially atmospheric stripping zone, compressing said mixture of hydrocarbons to a pressure equal to the pressure in said contacting zone and supplying said mixture of hydrocarbons to the contacting zone as the aforementioned mixture of light aromatic and nonaromatic hydrocarbons, withdrawing formamide from the bottom of said second substantially atmospheric stripping zone, compressing the formamide to the pressure in the contacting zone and supplying the formamide to the contacting zone as the aforemen tioned formamide.

10. Claim 9 further characterized in that a recycle stream of aromatic hydrocarbons is withdrawn from the separating zone, compressed to the pressure in said contacting zone and introduced into the lower portion of said contacting zone as a refluxing stream.

11. Claim 7 further characterized in that the aromatic hydrocarbons contain at least a substantial amount of benzene.

12. Claim 9 further characterized in that the aromatic hydrocarbons contain at least a substantial amount of enzene.

13. Claim 7 further characterized in that the aromatic hydrocarbons contain at least a substantial amount of toluene.

14. Claim 9' further characterized in that the aromatic hydrocarbons contain at least a substantial amount of toluene.

15. Claim 7 further characterized in that the ratio by weight of sulfur dioxide to ethylene glycol in the contacting zone is maintained between about 2 to l and 9 to l.

16. Claim 9 further characterized in that the ratio by Weight of sulfur dioxide to formarnide by weight in the contacting zone is maintained between about 2 to 1 and 9 to 1.

17. Claim 7 further characterized in that the compressed sulfur dioxide gas is passed in indirect heat exchange relationship through the lower portion of said separating zone whereby the latent heat of condensation of the compressed sulfur dioxide gas is Withdrawn to provide the latent heat of vaporization of the sulfur dioxide liquid in the separating zone.

18. Claim 9 further characterized in that the compressed sulfur dioxide gas is passed in indirect heat exchange relationship through the lower portion of said separating zone whereby the latent heat of condensation of the compressed sulfur dioxide gas is withdrawn to provide the latent heat of vaporization of the sulfur dioxide liquid in the separating zone.

References Cited in the file of this patent UNITED STATES PATENTS 2,069,173 Miller Jan. 26, 1937 2,146,679 Koenernann et al. Feb. 7, 1939 2,261,799 Franklin Nov. 4, 1941 2,288,853 Sowers July 7, 1942 2,721,164 Fenske Oct. 18, 1955 2,777,800 Mitchell et al. Jan. 15, 1957 Attest:

U IfIITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. $003,006 October 3, 1961 1 Alfred W. Francis rtified that error appears in ,the above numbered pat- It is hereby ce entrequiring correction and that the said Letters Patent should read as corrected below Column 5, line 27',- for ".tenary" read ternary column 6, TABL'E II sub-heading to column 4 thereof for "n-Hexane" read n-Heptane --'q same column 6, TABLElI\{, main heading to columns 6 and 7 thereoL- for toluene read benzene column 8, line 14 for "presence" read pressure Signed and sealed this 3rd day of April 1962.

(SEAL) DAVID L. LADD Commissioner of Pateni ERNEST w. swniERj Attesting Officer 4 

1. THE METHOD OF SEPARATING LIGHT AROMATIC HYDROCARBONS FROM NONAROMATIC HYDROCARBONS BOILING AT SUBSTANTIALLY THE SAME BOILING POINT WHICH COMPRISES CONTACTING A MIXTURE OF LIGHT AROMATIC HYDROCARBONS AND NONAROMATIC HYDROCARBONS BOILING AT SUBSTANTIALLY THE SAME BOILING POINT WITH A SOLVENT COMPRISING A MIXTURE OF SULFUR DIOXIDE AND A MATERIAL SELECTED FROM THE GROUP CONSISTING OF ETHYLENE GLYCOL AND FORMAMIDE, SEPARATING THE MATERIAL INTO A RAFFINATE PHASE CONTAINING SUBSTANTIALLY ALL OF THE NONAROMATIC HYDROCARBONS AND AN EXTRACT PHASE CONTAINING SUBSTANTIALLY ALL OF THE AROMATIC HYROCARBONS IN CONJUNCTION WITH THE MIXTURE OF SULFUR DIOXIDE AND THE MATERIAL SELECTED FROM THE GROUP CONSISTING OF ETHYLENE GLYCOL AND FORMAMIDE, SEPARATELY REMOVING SULFUR DIOXIDE FROM THE EXTRACT PHASE FOR REUSE IN THE PROCESS, SEPARATELY REMOVING THE MATERIAL SELECTED FROM THE GROUP CONSISTING OF ETHYLENE GLYCOL AND FORMAMIDE FROM THE EXTRACT PHASE FOR REUSE IN THE PROCESS, AND SEPARATELY REMOVING FROM THE EXTRACT PHASE AS PRODUCT SUBSTANTIALLY PURE LIGHT AROMATIC HYDROCARBONS FREE OF NONAROMATIC HYDROCARBONS AND FREE OF SOLVENT MATERIAL. 